Myositis

Introduction

Myositis refers to a group of inflammatory muscle diseases that lead to muscle weakness, pain, and impaired function. These disorders can significantly affect quality of life and may involve systemic symptoms in addition to muscle inflammation. Early recognition and management are crucial to prevent long-term disability and complications.

Definition and Classification

Myositis encompasses several subtypes, each with distinct clinical features, pathology, and prognosis. Accurate classification is important for diagnosis and treatment planning.

• Polymyositis: An inflammatory myopathy primarily affecting proximal muscles in adults, characterized by symmetric weakness and elevated muscle enzymes.
• Dermatomyositis: A subtype associated with characteristic skin manifestations, including heliotrope rash and Gottron’s papules, alongside muscle inflammation.
• Inclusion Body Myositis: A progressive muscle disease often affecting distal muscles and quadriceps, typically resistant to conventional immunosuppressive therapy.
• Necrotizing Autoimmune Myopathy: Severe muscle necrosis with minimal inflammation, frequently associated with autoantibodies and statin use.
• Juvenile Myositis: Myositis occurring in children, which may present with unique features including calcinosis and cutaneous involvement.

Anatomy and Histology of Skeletal Muscle

Skeletal muscle is composed of specialized contractile fibers organized to facilitate movement and maintain posture. Understanding muscle anatomy and histology is essential for recognizing the pathological changes seen in myositis.

• Muscle Fiber Types: Type I fibers are slow-twitch, fatigue-resistant fibers, while Type II fibers are fast-twitch and generate rapid, forceful contractions.
• Muscle Fascicles and Sarcomeres: Muscle fibers are bundled into fascicles, which contain repeating units called sarcomeres responsible for contraction. Connective tissue supports these structures and transmits force.
• Blood Supply and Innervation: Muscles receive rich vascularization for oxygen delivery and waste removal. Motor neurons provide innervation, allowing voluntary control and coordination.

Pathophysiology

Myositis results from immune-mediated processes that cause muscle inflammation, fiber damage, and impaired function. The exact mechanisms vary by subtype but share common pathological features.

• Immune-Mediated Mechanisms: Aberrant activation of the immune system targets muscle fibers, leading to tissue injury.
• Inflammatory Cell Infiltration: Lymphocytes and macrophages invade muscle tissue, contributing to inflammation and weakness.
• Muscle Fiber Necrosis and Regeneration: Damaged fibers undergo necrosis, followed by attempted regeneration, which may be incomplete or abnormal in chronic disease.
• Role of Cytokines and Autoantibodies: Pro-inflammatory cytokines amplify tissue damage, while specific autoantibodies are associated with distinct myositis subtypes and clinical manifestations.

Etiology and Risk Factors

Myositis is primarily an autoimmune disorder, though several factors contribute to its development. Identifying these factors helps in understanding disease mechanisms and guiding therapy.

• Autoimmune Disorders: Myositis often occurs in association with other autoimmune conditions, including systemic lupus erythematosus and scleroderma.
• Genetic Predisposition: Certain HLA types and genetic variants increase susceptibility to myositis.
• Infections: Viral infections, such as Coxsackievirus, HIV, or influenza, can trigger immune-mediated muscle inflammation.
• Medications and Toxins: Drugs like statins or other myotoxic agents may provoke necrotizing autoimmune myopathy.
• Environmental Triggers: Ultraviolet exposure, toxins, or occupational exposures may precipitate disease in susceptible individuals.

Clinical Features

Symptoms

Patients with myositis typically present with muscle-related complaints and systemic symptoms depending on the subtype.

• Proximal Muscle Weakness: Difficulty rising from a chair, climbing stairs, or lifting objects.
• Fatigue and Exercise Intolerance: Muscles tire quickly with minimal exertion.
• Muscle Pain or Tenderness: Aching or discomfort in affected muscles.
• Systemic Symptoms: Fever, weight loss, and malaise may accompany inflammation.

Signs

On examination, myositis may reveal characteristic findings that help differentiate subtypes.

• Muscle Atrophy: Chronic inflammation can lead to visible wasting of muscles, particularly in the proximal limbs.
• Skin Manifestations: Dermatomyositis shows heliotrope rash on eyelids and Gottron’s papules over joints.
• Reduced Reflexes: Mild to moderate decreases in deep tendon reflexes may be present.
• Functional Limitations: Difficulty performing tasks requiring proximal strength, such as combing hair or rising from a seated position.

Diagnostic Evaluation

Diagnosing myositis requires a combination of laboratory tests, imaging, and tissue analysis to confirm inflammation and rule out other causes of muscle weakness.

• Laboratory Investigations: Elevated creatine kinase (CK) and aldolase levels indicate muscle damage. Inflammatory markers such as ESR and CRP may also be raised.
• Autoantibody Testing: Presence of antinuclear antibodies (ANA), anti-Jo-1, and other myositis-specific antibodies helps classify subtypes and predict prognosis.
• Electromyography (EMG): Detects electrical abnormalities in affected muscles, showing patterns consistent with inflammatory myopathy.
• Muscle Biopsy: Confirms the diagnosis by revealing inflammatory cell infiltration, necrosis, or inclusion bodies depending on the subtype.
• Imaging: MRI can identify areas of muscle inflammation and edema, guiding biopsy and monitoring disease activity.

Complications

Myositis can lead to significant complications affecting multiple organ systems if not properly managed.

• Chronic Muscle Weakness and Disability: Persistent inflammation may result in permanent muscle impairment and functional limitations.
• Respiratory Muscle Involvement: Weakness of diaphragm and accessory muscles can lead to breathing difficulties and increased infection risk.
• Cardiac Involvement: Myocarditis, arrhythmias, and cardiomyopathy may occur, particularly in severe or untreated cases.
• Associated Malignancies: Dermatomyositis is linked with an increased risk of malignancies, necessitating appropriate cancer screening.

Treatment and Management

Pharmacological Therapy

Medical management aims to reduce inflammation, suppress the immune response, and preserve muscle function.

• Glucocorticoids: First-line therapy to rapidly control inflammation and improve muscle strength.
• Immunosuppressive Agents: Drugs such as methotrexate, azathioprine, or mycophenolate mofetil are used for long-term control or steroid-sparing purposes.
• Biologics: Intravenous immunoglobulin (IVIG) and rituximab may be considered in refractory cases or severe disease.

Non-Pharmacological Therapy

Supportive care plays a crucial role in maintaining mobility and quality of life.

• Physical therapy and rehabilitation to strengthen muscles, improve range of motion, and prevent contractures.
• Nutritional support to maintain muscle mass and address weight loss.
• Occupational therapy to assist with activities of daily living and adaptive strategies.

Prognosis

The outcome of myositis varies by subtype, disease severity, and response to treatment. Early diagnosis and intervention improve recovery and reduce complications.

• Factors Influencing Recovery: Age, baseline muscle strength, presence of autoantibodies, and systemic involvement affect prognosis.
• Long-Term Outcomes: Many patients achieve partial or complete recovery with therapy, though chronic weakness may persist in some cases, especially in inclusion body myositis.
• Impact of Early Diagnosis and Treatment: Prompt initiation of immunosuppressive therapy and rehabilitation is associated with better functional outcomes and lower complication rates.

Prevention and Patient Education

While primary prevention of myositis is not always possible due to its autoimmune nature, patient education and early intervention can reduce disease progression and complications.

• Monitoring for early symptoms such as unexplained muscle weakness, fatigue, or skin rashes.
• Avoidance of known triggers, including certain medications and environmental exposures.
• Adherence to prescribed therapy and regular follow-up to monitor disease activity and adjust treatment as needed.
• Engagement in physical activity and rehabilitation programs to maintain muscle strength and function.

References

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